This invention relates to an improvement in a metal vapor discharge lamp of the type comprising an arc tube, a starter connected in parallel thereto, and a translucent outer bulb housing them, for example, in a high-pressure sodium lamp having a starter therein.
FIG. 1 shows the equivalent circuit configuration of a conventional high-pressure sodium lamp having a starter therein along with an example of the lighting circuit thereof. The construction of this high-pressure sodium lamp is such that a translucent outer bulb 4 houses an arc tube 1 and a serial circuit which is connected in parallel thereto and which consists of a nonlinear capacitor 2 and a bimetal switch 3. This lamp is connected through a ballast 5 consisting of an inductor to an alternating current power source 6, the lamp being lit by applying an alternating voltage thereto. The nonlinear capacitor 2 then acts as a switching element in accordance with its voltage-charge hysteresis characteristic, rapidly interrupting the current flowing through the ballast 5. This causes a high-voltage pulse to be generated on either end of the ballast 5. The pulse is applied to the arc tube 1 along with the power source voltage, thereby lighting the lamp. After the lamp has come on, the bimetal switch 3 is opened by the heat from the arc tube 1, thereby stopping the operation of the starter.
The nonlinear capacitor adopted in a lamp of the type described above, is described in detail, for example, in Japanese Patent Publication No. 62-60803. FIGS. 2A and 2B show schematically the construction thereof. The capacitor shown comprises a dielectric substrate 7 which is principally made of barium titanate or the like. Provided on both sides of the dielectric substrate 7 are electrode films 8a and 8b to which lead wires 9a and 9b are electrically connected, respectively. The heat resistant material with which the electrode films 8a and 8b are covered and the details regarding the joint structure between the electrode films 8a, 8b and the lead wires 9a, 9b are omitted in the drawings.
When incorporating the nonlinear capacitor 2 having the above-described construction into a lamp, the lead wires 9a and 9b of the nonlinear capacitor 2 are often connected and fixed by means of welding or the like to support lines 10a and 10b of different polarities which are connected to conductive arc tube supports or the like, as shown in FIG. 3.
However, it has been found that lamps having the above-described construction involve the following problems in service:
(1) When an alternating electric field is applied to a nonlinear capacitor, inversion occurs in the spontaneous polarization thereof, so that the crystals constituting the dielectric substrate are elongated in the direction in which the electric field is applied, and are contracted in the direction perpendicular to the electric field, a phenomenon called electrostriction. As a result, a voltage-charge hysteresis characteristic is obtained.
However, if the lead wires 9a and 9b of the nonlinear capacitor 2 are firmly connected to the conductive supports 10a and 10b as shown in FIG. 3, the oscillation due to the electrostriction of the dielectric substrate 7 of the nonlinear capacitor 2 is restrained to a considerable degree, so that a satisfactory voltage-charge hysteresis characteristic cannot be obtained. As a result, the voltage pulse generated is inevitably rather low.
Moreover, it has been found that the oscillation due to the electrostriction of the dielectric substrate 7 involves a noise of a considerable magnitude since it is transmitted to other sections as the oscillation of the lead wires 9a, 9b and the conductive supports 10a, 10b.
(2) If the operation is continued while restraining the oscillation due to the electrostriction of the dielectric substrate 7 to a considerable extent as described above, an internal stress will act within the dielectric substrate 7 to generate cracks along the grain boundaries, thereby damaging the substrate. In some cases, discharge may occur between the electrodes of the nonlinear capacitor when the dielectric substrate suffers damage, which can cause a large electric current to flow through the starter, thereby burning the winding of the ballast 5.
(3) Discharge may occur between the electrodes of the nonlinear capacitor if near the end of the lamp service life the rare gas in the arc tube leaks into the outer bulb that houses the nonlinear capacitor, which will generate an atmosphere in the outer bulb that allows discharge to occur easily. As a result, a large current may flow through the starter, which also leads to burning of the ballast winding.